Many electronic devices, such as computers and many household appliances, require one or more regulated DC voltages. The power for such electronic devices is ordinarily supplied by power converters that convert an AC line voltage into the regulated DC voltages required by the devices.
Electrical power converters commonly include a rectifier circuit which converts the AC line voltage to an unregulated DC voltage, also known as a rectified line voltage, and a DC-to-DC converter for converting this unregulated DC voltage into one or more regulated DC output voltages. If a simple rectifier circuit is used, such power converters commonly draw high currents near the peak of the AC voltage cycle, and substantially zero current around the zero-crossing points of the voltage cycle. Thus, the input current drawn by the converter has a highly non-sinusoidal waveform with correspondingly high harmonic content.
As is known in the electrical power art, current harmonics above the fundamental frequency of the voltage do not contribute to the power drawn from a typical AC voltage source, with the result that the actual power drawn by the power supply is lower than the apparent power drawn. The explanation for this phenomenon is straightforward. The apparent power drawn is defined to be the product of the RMS voltage times the RMS current. The actual or "true" power is the integrated product of the instantaneous voltage and current, V*I, over a voltage cycle divided by the cycle's period. As is well-known, the integral of a fundamental harmonic with any other harmonic over one fundamental period is zero. Thus, assuming the input voltage is a sinusoid, the true power is simply the integral of the voltage sinusoid with the fundamental current harmonic, and the higher order current harmonics do not contribute to the true power. By contrast, all of the current harmonics contribute to the RMS value for the current; thus, the higher order harmonics do contribute to the apparent power drawn from the AC voltage source. Similarly, the apparent power is also higher than the true power when there is a phase difference between the voltage sinusoid and the fundamental current harmonic.
The distinction between apparent power and true power is important because power supplies are rated according to the apparent power drawn rather than the true power drawn. As a basis of comparison, the true power and apparent power drawn by a device are divided to form a ratio called the "power factor." Power factors less than about 80 percent can pose barriers to the performance or improvement of many types of electronic devices that operate on direct current, including such devices as personal computers, minicomputers, and appliances using microprocessors. For example, the high current peaks associated with low power factors can cause circuit breakers on the AC line to trip, which limits system design in terms of the functional load it places on the AC line. Additionally, the harmonics associated with the high, non-sinusoidal current peaks often result in power-line distortion, noise, and electromagnetic interference (EMI). In general, improving the power factor of the device reduces the harmonic content and electromagnetic noise.
To address these problems, many power supplies include power factor correction circuitry that is designed to raise power factors and eliminate harmonic distortion. Such circuits are often referred to as power factor correction circuits (abbreviated "PFC"). Power factor correction circuits generally rectify the AC line voltage and produce an unregulated DC voltage (referred to herein as the "PFC voltage") in a manner that has a relatively high power factor within a given range of AC line voltages. A switching power converter then converts the PFC voltage into the required regulated voltages.
It is generally desirable for power factor correction circuits to achieve adequate results over the range of AC line voltages that are standard in various pans of the world (i.e., 85 Vrms to 265 Vrms). It is well known that, in order to achieve adequate power factor correction with a conventional PFC, the PFC voltage generally must be greater than the peak AC line voltage. If the PFC voltage is lower than the peak AC line voltage, adequate power factor correction ordinarily will not occur and the waveform of the input current to the power converter will have high peaks similar to the peaks that would occur if the PFC were replaced by a simple bridge rectifier circuit. To ensure adequate power factor correction over the range of possible input voltages, the average value of the PFC voltage generally is either fixed at a relatively high value (e.g. in excess of 350 volts), or the average PFC voltage has a fixed relationship to the rms value of the AC line voltage (i.e., the PFC voltage is always higher than the RMS line voltage by a fixed factor or increment).
Applicant has discovered that the overall efficiency of the power converter is generally better when the average PFC voltage is lower. In other words, power losses within the power converter are generally greater when the PFC voltage is higher. Consequently, conventional power factor correction circuits having a high fixed PFC voltage can result in undesirably high power losses and correspondingly poor converter efficiency. Similarly, when the PFC voltage varies with the value of the AC line voltage, the PFC voltage is often unnecessarily high for the AC line voltage level, resulting in power losses and poor converter efficiency.
Aside from having relatively poor efficiency, conventional power factor corrected power converters often have additional problems related to unnecessarily high PFC voltages. The high power losses associated with high PFC voltages can result in undesirably high temperatures within the power converter. Additionally, operating the converter at such voltages can result in component failure and poor reliability.
An additional problem is that efficiency may need to be maximized under certain load conditions while power factor correction may be more important under other load conditions. For example, under light load conditions, it is often desirable or required to maximize efficiency without regard to power factor considerations. At higher loads, a high power factor may be required in order to draw sufficient input power. Conventional power factor correction circuitry generally operates in the same manner to correct the power factor under all load conditions. Thus, conventional power factor correction circuits do not accommodate the differences in power factor and efficiency requirements under different load conditions.
One approach to solving some of the problems associated with high PFC voltages might be to design power factor correction circuits that have PFC voltages that are as low as possible. Such power factor correction circuits, however, would not provide adequate power factor correction when the PFC voltage is too low in comparison to the value of the input AC line voltage.
Accordingly, there is a need for an electrical power converter system having a power factor correction circuit with a PFC voltage controlled such that the power converter system is efficient, while enabling adequate power factor correction to be maintained. There is a further need for an electrical power converter wherein the PFC voltage is controlled to reduce power dissipation when excessive temperatures occur or under certain load conditions.